Steel bar or wire rod for cold forging and method of producing the same

ABSTRACT

A bar or wire product for use in cold forging, characterized in that it comprises a steel having the chemical composition, in mass %: C: 0.1 to 0.6%, Si: 0.01 to 0.5%, Mn: 0.2 to 1.7%, S: 0.001 to 0.15%, Al: 0.015 to 0.05%, N: 0.003 to 0.025%, P: 0.035% or less, O: 0.003% or less and balance: Fe and inevitable impurities, and it has, in the region from the surface thereof to the depth of the radius thereof×0.15, a structure wherein ferrite accounts for 10 area % or less and the balance is substantially one or more of martensite, bainite and pearlite, and the average hardness in the region from the depth of the radius thereof×0.5 to the center thereof is less than that of the surface layer thereof by 20 or more of HV; and a method for producing the bar or wire product. The bar or wire product is excellent in the ductility after spheroidizing and thus allows the prevention of occurrence of cracks in a steel product during cold forging, which has conventionally been a problem in manufacturing structural parts for a machine by cold forging.

TECHNICAL FIELD

The present invention relates to a steel bar or wire rod, for coldforging, used for manufacturing machine structural components such asthe components of cars, construction machines and the like, and to amethod of producing the same and, more specifically, to a steel bar orwire rod, for cold forging, excellent in ductility and thus beingsuitable for heavy cold forging work, and a method of producing thesame.

BACKGROUND ART

Carbon steels for machine structural use and low alloy steels formachine structural use have been used conventionally as the structuralsteels for the manufacture of machine structural components such as thecomponents of cars, construction machines and the like. The machinestructural components for cars such as bolts, rods, engine componentsand driving system components have so far been manufactured from thesesteel materials mainly through a hot forging and machining process.However, the recent trend is that the above hot forging and machiningprocess is replaced with a cold forging process in view of advantagessuch as the improvement of productivity. In a cold forging process, coldforging work is usually applied to a hot rolled steel material after itis subjected to spheroidizing annealing (SA) and cold workability issecured. A problem here is that the cold forging causes work hardeningof the steel material and its ductility is lowered, resulting in theoccurrence of cracks and a shorter service life of metal dies. Theoccurrence of cracks during the cold forging work, or the insufficiencyof steel ductility, often constitutes the main obstacle in the changefrom a hot forging process to a cold forging process, especially whenheavy cold forging is required.

Meanwhile, in the spheroidizing annealing (SA), a steel material has tobe heated to a high temperature and held there for a long time and,consequently, an apparatus for heat treatment such as a heating furnaceis required and, in addition, energy is consumed for the heating and,for this reason, the spheroidizing annealing is responsible for a largeproportion of the manufacturing cost. In view of the above, varioustechnologies, such as those described below, have been proposed for thepurposes of enhancing productivity, saving energy, etc.

For the purpose of reducing the time for the spheroidizing annealing,Japanese Unexamined Patent Publication No. S57-63638 proposes a methodfor obtaining a steel wire rod excellent in cold forging properties bycooling a hot-rolled steel material to 600° C., at a cooling rate of 4°C./sec. or higher, to form a quenched structure and then applyingspheroidizing annealing to the steel material covered with scale in aninert gas atmosphere. For enabling quick spheroidizing, JapaneseUnexamined Patent Publication No. S60-152627 discloses a method in whichfinish rolling conditions are specifically defined and a steel materialis rapidly cooled after the rolling to obtain a structure where finepearlite, bainite or martensite is mixed in finely dispersedpro-eutectoid ferrite. Japanese Unexamined Patent Publication No.S61-264158 proposes a method for lowering the steel hardness afterspheroidizing annealing by improving the chemical composition of asteel, namely by obtaining a low carbon steel wherein the content of Pis reduced to 0.005% or less and the expressions Mn/S≧1.7 and Al/N≧4.0are satisfied. Japanese Unexamined Patent Publication No. S60-114517proposes a method in which controlled rolling is applied for the purposeof eliminating a softening annealing process before cold working.

All these conventional technologies aim at improving or eliminating thespheroidizing annealing before the cold forging work and do not aim atimproving the insufficient ductility of steel materials, whichconstitutes the main obstacle in the change from a hot forging processto a cold forging process in the manufacture of machine componentsrequiring heavy working.

DISCLOSURE OF THE INVENTION

In view of the above situation, the object of the present invention isto provide a steel bar or wire rod for cold forging excellent inductility after spheroidizing annealing, capable of preventing, in themanufacture of machine structural components from a hot-rolled steel baror wire rod through spheroidizing annealing and cold forging, theconventional problem of cracking of a steel material during cold forgingwork, and a method of producing the same.

As a result of investigations into the cold workability of a steel baror wire rod for cold forging, the inventors of the present inventiondiscovered that it was possible to obtain a steel bar or wire rod forcold forging excellent in ductility after spheroidizing annealing byhardening only the surface layer of a steel bar or wire rod having aspecific chemical composition and forming a soft structure in its centerportion.

The gist of the present invention, which has been established on thebasis of the above finding, is as follows:

-   -   (1) A steel bar or wire rod for cold forging excellent in        ductility after spheroidizing annealing, characterized by:        consisting of a steel containing, in mass,        -   0.1 to 0.6% of C,        -   0.01 to 0.5% of Si,        -   0.2 to 1.7% of Mn,        -   0.001 to 0.15% of S,        -   0.015 to 0.05% of Al and        -   0.003 to 0.025% of N,            and having the contents of P and O controlled to 0.035% or            less and 0.003% or less, respectively, with the balance            consisting of Fe and unavoidable impurities; the area            percentage of ferrite in the metallographic structure of the            portion from the surface to the depth of 0.15 of its radius            being 10% or less, with the rest of the structure consisting            substantially of one or more of martensite, bainite and            pearlite; and the average hardness of the portion from the            depth of 0.5 of its radius to the center being lower than            that of its surface layer (the portion from the surface to            the depth of 0.15 of the radius) by HV 20 or more.    -   (2) A steel bar or wire rod for cold forging excellent in        ductility after spheroidizing annealing according to the item        (1), characterized by further containing, in mass, one or more        of 3.5% or less of Ni, 2% or less of Cr and 1% or less of Mo.    -   (3) A steel bar or wire rod for cold forging excellent in        ductility after spheroidizing annealing according to the        item (1) or (2), characterized by further containing, in mass,        one or more of 0.005 to 0.1% of Nb and 0.03 to 0.3% of V.    -   (4) A steel bar or wire rod for cold forging excellent in        ductility after spheroidizing annealing according to any one of        the items (1) to (3), characterized by further containing, in        mass, one or more of 0.02% or less of Te, 0.02% or less of Ca,        0.01% or less of Zr, 0.035% or less of Mg, 0.1% or less of Y and        0.15% or less of rare earth elements.    -   (5) A steel bar or wire rod for cold forging excellent in        ductility after spheroidizing annealing according to any one of        the items (1) to (4), characterized in that the austenite grain        size number according to Japanese Industrial Standard (JIS) in        the portion from the surface to the depth of 0.15 of its radius        is 8 or higher.    -   (6) A method of producing a steel bar or wire rod for cold        forging excellent in ductility after spheroidizing annealing,        characterized by: finish-rolling a steel material having a        chemical composition specified in any one of the items (1)        to (5) while controlling its surface temperature to 700 to        1,000° C. at the exit from the final finish rolling stand,        during hot rolling, and, after that, subjecting the rolled        material to at least a process cycle of “rapidly cooling the hot        rolled material to a surface temperature of 600° C. or below and        subsequently making it recuperate by the sensible heat thereof        so that the surface temperature becomes 200 to 700° C.” or        repeating the process cycle twice or more; and, by doing so,        making the area percentage of ferrite in the structure of the        portion of the steel bar or wire rod from the surface to the        depth of 0.15 of its radius 10% or less, and the rest of the        structure consist substantially of one or more of martensite,        bainite and pearlite, and also, forming the structure in which        the average hardness of the portion from the depth of 0.5 of its        radius to the center is lower than that of its surface layer        (the portion from the surface to the depth of 0.15 of the        radius) by HV 20 or more.    -   (7) A steel bar or wire rod for cold forging excellent in        ductility characterized by: being a steel bar or wire rod        according to any one of the items (1) to (5) having undergone        spheroidizing annealing; the degree of spheroidized structure        according to JIS G 3539 in the portion from the surface to the        depth of 0.15 of its radius being No. 2 or below; and the degree        of spheroidized structure in the portion from the depth of 0.5        of its radius to the center being No. 3 or below.    -   (8) A steel bar or wire rod for cold forging excellent in        ductility according to the item (7), characterized in that the        ferrite grain size number under JIS in the portion from the        surface to the depth of 0.15 of its radius is 8 or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the position (mm) in asection of a steel bar 36 mm in diameter for cold forging according tothe present invention and the hardness (HV) at the position.

FIG. 2(a) is a micrograph (×400) of the surface of a steel bar and FIG.2(b) a micrograph (×400) of the center portion thereof.

FIG. 3(a) is a micrograph (×400) of the surface of a steel bar obtainedthrough the spheroidizing annealing of the steel bar shown in FIG. 1,and FIG. 3(b) a micrograph (×400) of the center portion thereof.

FIG. 4 is a schematic illustration showing the example of a rolling lineemployed for the present invention.

FIG. 5(a) is a diagram showing CCT curves to explain the structures inthe surface layer and the center portion of a steel bar or wire rod, andFIG. 5(b) a sectional view showing the structure of a steel bar or wirerod after cooling and recuperating.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be explained in detail hereafter.

In the first place, the reasons are given as to why the steel chemicalcomposition necessary for achieving the structure and the mechanicalproperties such as the hardness and ductility of a steel bar or wire rodfor cold forging, which are targeted in the present invention, isspecified.

-   -   C: C is an element indispensable for the enhancement of the        steel strength required of machine structural components. With a        C content less than 0.1%, the strength of a final product is        insufficient but, with a C content in excess of 0.6%, the        ductility of a final product is deteriorated. The C content is,        therefore, limited to 0.1 to 0.6%.    -   Si: Si is added as a deoxidizing agent and also for the purpose        of increasing the strength of a final product through solid        solution hardening. A content of Si below 0.01% is insufficient        for obtaining the above effects. However, when it is added in        excess of 0.5%, these effects do not increase any more and,        rather, the ductility is deteriorated. For this reason, the        content of Si is defined to be 0.01 to 0.5%. It is, however,        preferable to set the upper limit of the Si content at 0.35% or        lower or, more preferably, at 0.2% or lower.    -   Mn: Mn is an element effective for increasing the strength of a        final product through the enhancement of hardenability. With a        Mn content less than 0.2%, a sufficient effect is not obtained        and, with its addition in excess of 1.7%, not only the effect        becomes saturated but also ductility is deteriorated. The Mn        content is, therefore, limited to 0.2 to 1.7%.    -   S: S is a component inevitably included in steel and exists        there in the form of MnS. Its content is defined in the present        invention to be 0.001 to 0.15% because S is effective for        enhancing machinability and fining a crystal structure. However,        as S is detrimental to cold forming work, it is preferable to        limit its content to 0.015% or lower or, more preferably, to        0.01% or lower, when machinability is not required.    -   Al: Al is effective as a deoxidizing agent. It is also effective        for fining crystal grains by fixing solute N in steel as AlN.        With an excessive content of Al, however, an excessive amount of        Al₂O₃ is formed, resulting in an increase of internal defects        and the deterioration of cold workability. The content of Al is        therefore limited within the range from 0.015 to 0.05% in the        present invention.    -   N: N reacts with Al or Nb to form AlN or NbN (NbCN), fines        crystal grains and enhances steel ductility and, for this        reason, its content is set at 0.003 to 0.025%.    -   P: P is a component inevitably included in steel and causes        grain boundary segregation and center segregation, deteriorating        ductility. It is, therefore, desirable to limit the content of P        to 0.035% or less or, preferably, 0.02% or less.    -   O: O is a component inevitably included in steel too, and        deteriorates cold workability by reacting with Al to form Al₂O₃.        It is therefore desirable to control its content to 0.003% or        lower or, preferably, 0.002% or lower.

The basic chemical composition of a steel to which the present inventionis applied is as explained above. Further, in the present invention, asteel may contain one or more of Ni, Cr and Mo. These elements are addedfor increasing the strength of a final product through the enhancementof hardenability and similar effects. An addition of each of theseelements in a great quantity, however, causes bainite and martensite toform down to the center portion of an as hot-rolled steel bar or wirerod, raising steel hardness, and is not desirable from the economicalviewpoint, either. The contents of these elements, therefore, arelimited to 3.5% or less for Ni, 2% or less for Cr, and 1% or less forMo.

Yet further, in the present invention, for the purpose of controllingthe crystal grain size, Nb and/or V may be added to a steel. When thecontent of Nb is below 0.005% or that of V is below 0.03%, however, atangible effect is not obtained. On the other hand, when their contentsexceed 0.1 and 0.3%, respectively, the effect is saturated and, rather,the ductility is deteriorated. Hence, their contents are defined to be0.005 to 0.1% for Nb and 0.03 to 0.3% for V.

In addition, in the present invention, for the purposes of controllingthe shape of MnS, preventing cracks and enhancing ductility, a steel maycontain one or more of the following elements: 0.02% or less of Te,0.02% or less of Ca, 0.01% or less of Zr, 0.035% or less of Mg, 0.15% orless of rare earth elements, and 0.1% or less of Y. These elements formrespective oxides, and the oxides not only act as nuclei for theformation of MnS but also reform MnS into (Mn, Ca)S, (Mn, Mg)S, etc.This makes the sulfides easily stretchable during hot rolling, causinggranular MnS to disperse in fine grains, which increases ductility aswell as the critical upsetting ratio during cold forging work. On theother hand, when Te is added in excess of 0.02%, Ca in excess of 0.02%,Zr in excess of 0.01%, Mg in excess of 0.035%, Y in excess of 0.1%, orrare earth elements in excess of 0.15%, the above effects are saturatedand, adversely, CaO, MgO and other coarse oxides and the clusters ofthese oxides are formed, and hard compounds such as ZrN and the likeprecipitate, deteriorating ductility. For this reason, the contents ofthese elements are defined to be 0.02% or less for Te, 0.02% or less forCa, 0.01% or less for Zr, 0.035% or less for Mg, 0.1% or less for Y, and0.15% or less for rare earth elements. Note that the rare earth elementsdescribed in the present invention mean elements having atomic numbersof 57 to 71.

Here, the Zr content in steel is determined by the inductively coupledplasma emission spectrometry (ICP), in a manner similar to thedetermination of the content of Nb in steel, after a sample is treatedin the same manner as specified in Attachment 3 of JIS G 1237-1997. Theamount of each sample used in the measurement of Example of the presentinvention was 2 g per steel grade and a calibration curve for the ICPwas set so as to be suited for measuring a very small quantity of Zr.That is to say, solutions having different Zr concentrations wereprepared by diluting a standard solution of Zr so that the Zrconcentrations varied from 1 to 200 ppm, and the calibration curve wasdetermined by measuring the amounts of Zr in the diluted solutions. Notethat the common procedures related to the ICP are based on JIS K0116-1995 (General Rules for Emission Spectrometry) and JIS Z 8002-1991(General Rules for Tolerances of Tests and Analyses).

Next, the structure of a steel bar or wire rod according to the presentinvention is explained hereafter.

The present inventors studied methods of enhancing the ductility of asteel bar or wire rod for cold forging and made it clear that the key toenhancing the ductility of a spheroidizing-annealed steel material wasto make the spheroidizing-annealed structure homogeneous and fine, andthat, for this end, it was effective to control the percentage offerrite in the structure after hot rolling to a specified figure or lessand to make the balance a mixed structure consisting of one or more offine martensite, bainite and pearlite. It follows that the ductility ofa steel bar or wire rod increases when it is rapidly cooled after finishhot rolling and then spheroidizing-annealed. If it is rapidly cooled soas to harden the structure throughout its section, however, quenchingcracks are likely to occur and, besides, steel hardness does notdecrease even after the spheroidizing annealing and cold deformationresistance increases, which makes the service life of cold forging diesshorter. The present inventors discovered: that, for solving the aboveproblem, it was effective to temper the martensite formed in the surfacelayer of a steel bar or wire rod by rapidly cooling the surface layerafter finish hot rolling and subsequently making it recuperate by thesensible heat thereof and, by doing so, to soften the surface layerprior to spheroidizing annealing, and further to make the internalportion composed of a soft structure by making use of the low coolingrate; and that, as a result of the above, a steel bar or wire rod forcold forging excellent in ductility after spheroidizing annealing andhaving a low cold deformation resistance could be obtained.

FIG. 1 is a graph showing the relation between the position (mm, 0 atthe center) in a section of a steel bar 36 mm in diameter for coldforging according to the present invention and the hardness (HV) at theposition.

As seen in FIG. 1, the average hardness at the surface is HV 280 to 330and that at the center is roughly HV 200, and the hardness decreasesgradually towards the center.

As seen in the micrograph (×400) of the surface of the steel bar in FIG.2(a) and that of the center in FIG. 2(b), the structure at the surfaceconsists mainly of tempered martensite and that at the center mainly offerrite and pearlite.

As for the structure after the spheroidizing annealing to hold the steelbar shown in FIG. 1 at 735° C. for 1 h. and then at 680° C. for 2 h., asis clear from the micrograph (×400) of the surface of the steel bar inFIG. 3(a) and that of the center in FIG. 3(b), a homogeneous structurehaving a good degree of spheroidizing is obtained at the surface. Thehardness after the spheroidizing annealing is about HV 135, roughly thesame from the surface to the center.

Even though a steel bar after spheroidizing annealing is subjected to anupsetting test under heavy working of a true strain exceeding 1, it didnot develop any cold forging cracks and its cold deformation resistanceremained at a low level not causing any problem during cold forgingwork.

Based on this result, the present inventors further proceeded with testsand examinations into the structure of the surface layer and therelation between the hardness of the surface layer and that of thecenter portion not causing cracking at cold forging work.

As a result, the present inventors discovered: that, even if the surfacelayer was composed of a tempered martensite structure (a structure inwhich ferrite exists in a phase consisting substantially of one or moreof martensite, bainite and pearlite), the cold forging cracks could notbe prevented from occurring unless the area percentage of ferrite was10% or less in the portion of a steel bar or wire rod from the surfaceto the depth of 0.15 of its diameter, or, preferably 5% or less in thecase of heavy cold forging work; that, in order to secure the ductilityduring cold forging and prevent cracks from occurring and deformationresistance from increasing, it was necessary to form a fine andhomogeneous structure having a higher percentage of tempered martensitein the surface layer at the stage after the steel bar or wire rod washot-rolled; and that, for this end, it was necessary to createdifference in hardness between the surface layer and the center portionat the stage after the steel bar or wire rod was hot-rolled and thenecessary condition for achieving the above was to make the averagehardness (HV) of the portion from the depth of 0.5 of the radius of thesteel bar or wire rod to its center lower than the average hardness (HV)of the portion from the surface to the depth of 0.15 of the radius by HV20 or more, or, preferably by HV 50 or more in the case of heavy coldforging work.

Then, when the steel bar or wire rod described above was subjected tospheroidizing annealing (SA), a steel bar or wire rod for cold forgingexcellent in ductility was obtained, wherein the degree of spheroidizedstructure defined by JIS G 3539 in the portion of the steel bar or wirerod from the surface to the depth of 0.15 of its radius was No. 2 orbelow. It was confirmed that the spheroidizing-annealed steel bar orwire rod thus obtained did not develop cold forging cracks even thoughit was subjected to an upsetting test under heavy working of a truestrain exceeding 1.

Note that the conventionally known methods of spheroidizing annealingcan be employed for the spheroidizing annealing of the presentinvention.

In order to obtain the grain size of the crystals in the surface layercontributing to the enhancement of ductility, at the stage before thespheroidizing annealing, it is enough to make the austenite crystalgrain size number under JIS G 0551 not less than 8 in the portion of thesteel bar or wire rod from the surface to the depth of 0.15 of itsradius. Here, it is preferable to make the number not less than 9 whenbetter properties are required, or not less than 10 when still higherproperties are required. Then, at the stage after the spheroidizingannealing, it is enough to make the ferrite crystal grain size numberunder JIS G 3545 not less than 8 in the portion of the steel bar or wirerod from the surface to the depth of 0.15 of its radius, and it ispreferable to make the number not less than 9 when better properties arerequired, or not less than 10 when still higher properties are required.

When the crystal grain size numbers are not more than the numbersspecified above, sufficient ductility is not achieved.

Next, a method of producing the steel bar or wire rod for cold forgingaccording to the present invention is explained hereafter.

FIG. 4 is a schematic illustration showing the example of a rolling lineemployed in the present invention.

As seen in FIG. 4, a steel having a chemical composition according toany one of claims 1 to 5 is heated in a reheating furnace 1 andfinish-rolled through a hot rolling mill 2 so that the surfacetemperature of the steel bar or wire rod is controlled to 700 to 1,000°C. at the exit from the final finish rolling stand. The temperature atthe exit from the final finish rolling stand is measured with apyrometer 3. Then, the finish-rolled steel bar or wire rod 4 is rapidlycooled by applying water to the surface in the cooling troughs 5(preferably, at an average cooling rate of 30° C./sec. or higher, forexample) to a surface temperature of 600° C. or lower, preferably 500°C. or lower or, more preferably 400° C. or lower, so that the structureof the surface layer consists mainly of martensite. After passingthrough the cooling troughs, the surface layer of the steel bar or wirerod is recuperated by the sensible heat of its center portion to asurface temperature of 200 to 700° C. (measured with a pyrometer 6) sothat the structure of the surface layer consists mainly of temperedmartensite.

In the present invention, the above rapid cooling and recuperatingprocess is conducted at least once or more. This remarkably enhances theductility of a steel.

The reason why the surface temperature of the steel bar or wire rod iscontrolled to 700 to 1,000° C. is that crystal grains can be made finethrough low temperature rolling and, by so doing, the structure afterthe rapid cooling can be made fine: when the surface temperature is1,000° C. or lower, the austenite grain size number in the surface layerbecomes 8; when it is 950° C. or lower, the number becomes 9; and whenit is 860° C. or lower, the number becomes 10. When the surfacetemperature is below 700° C., however, it becomes difficult to reducethe quantity of ferrite in the structure of the surface layer, and, forhis reason, the surface temperature must be 700° C. or above.

Note that a method and an apparatus of such direct surface quenching(DSQ) are publicly known as disclosed in Japanese Unexamined PatentPublication Nos. S62-13523 and H1-25918, though the object to which theyare applied is other than that of the present invention.

FIG. 5 is a diagram showing CCT curves for explaining the structures ofthe surface layer and the center portion of a steel bar or wire rod.

As shown in the figure, when a steel bar or wire rod finish-rolled at alow temperature is rapidly cooled and then recuperated, the structure ofthe surface layer 7, which is cooled at a high cooling rate, mainlyconsists of tempered martensite, while that of the center portion 8,which is cooled at a lower cooling rate than the surface layer, consistsof ferrite and pearlite.

The reason why a steel bar or wire rod is rapidly cooled to a surfacetemperature of 600° C. or below and then it is recuperated by thesensible heat to a surface temperature of 200 to 700° C. is to make thesurface layer consist of a structure mainly composed of temperedmartensite and having a reduced hardness.

EXAMPLE

Examples of the present invention are explained hereafter.

The steels listed in Table 1 were rolled into steel bars and wire rodsunder the rolling conditions listed in Table 2. The diameter of therolled products ranged from 36 to 55 mm. After that, the steel bars andwire rods underwent spheroidizing annealing and then a hardeningtreatment through quenching and tempering. The structures and propertiesof the steel bars and wire rods were investigated at the stages rightafter rolling, after spheroidizing-annealing and after quenched andtempered, respectively. The results are shown in Tables 3 and 4. “Theportion of a steel bar or wire rod from the surface to the depth of 0.15of the radius” referred to in the claims of the present invention isexpressed in Tables 3 and 4 simply as “surface layer” (e.g., surfacelayer hardness). Likewise, “the portion of a steel bar or wire rod fromthe depth of 0.5 of the radius to the center” referred to in the claimsof the present invention is expressed in the tables simply as “centerportion” (e.g., center portion hardness). The deformation resistance ofeach of the steel bars and wire rods was measured by subjecting thecolumnar test piece having the same diameter as the rolled product and aheight 1.5 times the diameter to the upsetting test. A criticalupsetting ratio was measured by subjecting each of the columnar testpieces of the aforementioned dimension, each having a notch 0.8 mm indepth and 0.15 mm in notch apex radius at the surface, to the upsettingtest. The test pieces for tensile test were cut out from the positionscorresponding to the surface layers of the rolled products, and thetensile strength and reduction of area, which is an indicator ofductility, of the surface layers were measured through tensile test. Therolled products of each steel underwent any one of the common quenchingand tempering (common QT), induction quenching and tempering (IQT) andcarburizing quenching and tempering (CQT). The induction quenching wasconducted at a frequency of 30 kHz. The carburizing quenching wasconducted under the condition of a carbon potential of 0.8% and 950°C.×8 h.

TABLE 1 (mass %) Steel C Si Mn S Al N P O Ni Cr Mo Nb V Te Ca 1 0.250.23 0.47 0.008 0.028 0.0035 0.020 0.0014 — — — — — — — 2 0.25 0.20 1.100.009 0.031 0.0051 0.009 0.0008 — — — — — — — 3 0.34 0.22 0.80 0.0190.029 0.0042 0.014 0.0014 — — — — — — — 4 0.40 0.24 0.82 0.009 0.0300.0043 0.012 0.0007 — — — — — — — 5 0.45 0.29 0.78 0.008 0.030 0.00510.012 0.0009 — — — — — — — 6 0.48 0.25 0.80 0.008 0.026 0.0048 0.0080.0013 — — — — — — — 7 0.53 0.29 0.74 0.009 0.027 0.0050 0.009 0.0009 —— — — — — — 8 0.35 0.29 1.28 0.013 0.028 0.0047 0.009 0.0007 — — — — — —— 9 0.40 0.22 1.38 0.008 0.027 0.0045 0.024 0.0009 — — — — — — — 10 0.460.23 1.21 0.012 0.025 0.0052 0.012 0.0012 — — — — — — — 11 0.53 0.211.08 0.011 0.033 0.0048 0.014 0.0008 — — — — — — — 12 0.33 0.05 0.650.009 0.027 0.0043 0.008 0.0008 — 0.30 — — — — — 13 0.40 0.04 0.67 0.0120.028 0.0045 0.013 0.0014 — 0.45 — — — — — 14 0.44 0.05 0.64 0.008 0.0290.0051 0.010 0.0010 — 0.31 — — — — — 15 0.53 0.04 0.65 0.009 0.0310.0047 0.014 0.0009 0.51 16 0.40 0.25 0.82 0.009 0.030 0.0054 0.0120.0013 — 1.06 — — — — — 17 0.35 0.23 0.79 0.007 0.028 0.0046 0.0130.0015 — 1.03 0.17 — — — — 18 0.32 0.27 1.31 0.007 0.028 0.0105 0.0150.0014 — — — — 0.15 — — 19 0.43 0.23 1.41 0.008 0.030 0.0051 0.0120.0011 — 0.12 — — — 0.0030 — 20 0.48 0.23 0.77 0.007 0.028 0.0058 0.0120.0014 — — — — — 0.0023 — 21 0.35 0.24 0.81 0.013 0.027 0.0058 0.0130.0014 — 1.01 0.16 — — 0.0024 — 22 0.15 0.22 0.80 0.013 0.029 0.01340.014 0.0013 — 1.10 0.16 — — — — 23 0.20 0.24 0.82 0.010 0.030 0.01520.012 0.0007 — 1.12 — — — — — 24 0.15 0.23 0.51 0.008 0.029 0.0142 0.0120.0012 2.24 0.41 — — — — — 25 0.20 0.22 0.83 0.008 0.028 0.0152 0.0100.0009 0.51 0.49 0.17 — — — — 26 0.20 0.05 0.65 0.009 0.031 0.0148 0.0120.0010 — 1.59 — — — — — 27 0.15 0.04 0.64 0.007 0.029 0.0140 0.0130.0012 — 1.55 0.16 — — — — 28 0.20 0.23 0.84 0.009 0.030 0.0149 0.0130.0011 — 1.12 — 0.021 — — — 29 0.19 0.24 0.81 0.008 0.029 0.0152 0.0140.0010 — 1.11 0.16 0.025 — — — 30 0.20 0.21 0.79 0.008 0.029 0.01520.013 0.0012 — 1.12 0.17 0.019 0.10 — — 31 0.19 0.04 0.63 0.010 0.0300.0145 0.013 0.0010 — 1.60 — 0.024 — — — 32 0.20 0.04 0.65 0.009 0.0290.0147 0.011 0.0012 — 1.57 0.16 0.020 — 33 0.20 0.04 0.65 0.008 0.0290.0148 0.011 0.0010 0.51 0.72 0.10 0.0030 34 0.19 0.23 0.79 0.008 0.0290.0147 0.012 0.0009 1.13 0.03 0.022 — 0.0025 —

TABLE 2 Steel surface Number of Surface temperature RecuperationReference temperature at repetitions of immediately after temperaturesymbol of exit from rapid cooling rapid cooling (Average rolling finishrolling and recuperating (Average temperature temperature Classificationconditions stand, ° C. cycle in II) in II) Invented I 790-940 1 cycleRoughly 100° C. 400-590° C. examples II 770-920 7 Roughly 500° C.380-650 Comparative III 870-940 Air-cooled after hot rolling examples

TABLE 3 Structure and properties of bar Structure and prop- or wire roderties after spheroid- Hardness izing annealing difference Degree Degreebetween of sphe- of sphe- Area surface γ grain roidized roidized Roll-percentage Surface Center layer and size struc- struc- Refer- ing offerrite layer portion center number of ture of ture of Classifi- enceSteel condi- in surface hardness, hardness, portion, surface surfacecenter cation symbol No. tion layer, % HV HV HV layer layer portionRange ≦10% ≧20% ≧ No. 8 ≦ No. 2 ≦ No. 3 specified in the presentinvention Example 1  1 I 4 223 167 56 of first 2  3 I 3 282 220 62invention 3  6 I 0 290 225 65 4 11 II 0 319 248 71 Example 5 13 I 0 292225 67 of second 6 15 I 0 330 242 88 invention Example 7 18 I 0 317 25463 of third invention Example 8 19 I 0 294 224 70 of fourth inventionExample 9 25 I 0 365 256 109  of second 10 26 I 0 340 231 110  inventionExample 11 28 I 0 345 242 103  of third 12 32 I 3 297 220 77 inventionExample 13 33 I 0 322 234 88 of fourth invention Example 14  4 I 0 293226 67  9.7 of fifth 15  7 I 0 332 245 87 10.8 invention 16  9 I 0 304231 73  9.5 17 17 I 0 281 219 63 10.4 18 20 I 0 290 223 67  9.9 19 22 I0 343 242 101  11.8 20 30 II 0 295 225 70  9.2 Structure and propertiesafter spheroidizing annealing Ferrite grain size Surface number Defor-Surface Reduc- hardness Refer- of mation Critical layer Tensile tionafter QT, HV Classifi- ence surface resistance, upsetting hardness,strength, of area, Common cation symbol layer MPa ratio, % HV MPa % QTIQT CQT Range ≧ No. 8 specified in the present invention Example  1 66057.4 130 400 91 230 of first  2 690 52.2 139 465 84 620 invention  3 75050.5 146 533 73 650  4 780 48.2 154 572 68 692 Example  5 773 50.0 143521 77 653 of second  6 792 46.3 160 584 67 700 invention Example  7 77848.6 154 570 67 624 of third invention Example  8 752 50.8 145 533 73653 of fourth invention Example  9 687 55.2 135 462 76 812 of second 10665 57.4 132 457 87 809 invention Example 11 674 56.8 134 455 88 778 ofthird 12 675 56.4 132 461 85 780 invention Example 13 681 57.6 135 45986 805 of fourth invention Example 14 774 50.2 149 521 77 656 of fifth15 793 46.2 162 583 68 698 invention 16 766 51.2 139 516 78 662 17 69252.3 140 453 83 618 18 749 51.3 145 532 75 653 19 677 57.2 136 453 87802 20 674 56.6 134 462 83 795 Common QT: Quenching after heating to900° C. and tempering at 550° C.; IQT: induction quenching and temperingat 170° C.; CQT: carburization quenching and tempering at 170° C.

TABLE 4 Structure and properties of bar Structure and prop- or wire roderties after spheroid- Hardness izing annealing difference Degree Degreebetween of sphe- of sphe- Area surface γ grain roidized roidized Roll-percentage Surface Center layer and size struc- struc- Refer- ing offerrite layer portion center number of ture of ture of Classifi- enceSteel condi- in surface hardness, hardness, portion, surface surfacecenter cation symbol No. tion layer, % HV HV HV layer layer portionRange ≦10% ≧20% ≧ No. 8 ≦ No. 2 ≦ No. 3 specified in the presentinvention Example 21  2 I 0 281 220 61 1 2 of 24 10 I 0 292 223 69 1 2seventh 25 12 I 0 284 221 63 1 2 invention 27 16 I 0 295 227 68 1 2 2923 I 0 361 252 109  1 2 31 27 I 0 343 230 113  1 2 33 31 II 0 315 230 851 2 Example 22  5 I 0 286 205 81 1 2 of eighth 23  8 I 0 284 219 65 1 2invention 26 14 I 0 287 206 81 1 2 28 21 I 0 318 225 93 1 2 30 24 I 0357 243 114  10.4 1 2 32 29 II 0 360 258 102  1 2 34 34 I 0 345 240 105  9.8 1 2 Compara- 35  5 III 45  186 180  6 3 4 tive 36 23 III 54  195187  8 3 4 examples 37 22 III 26  230 221  9 3 3 Structure andproperties after spheroidizing annealing Ferrite grain size Surfacenumber Defor- Surface Reduc- hardness Refer- of mation Critical layerTensile tion after QT, HV Classifi- ence surface resistance, upsettinghardness, strength, of area, Common cation symbol layer MPa ratio, % HVMPa % QT IQT CQT Range ≧ No. 8 specified in the present inventionExample 21 658 58.8 132 402 90 233 of 24 778 49.4 157 563 70 682 seventh25 689 53.1 140 463 83 622 invention 27 772 50.4 142 523 79 659 29 68555.8 133 458 87 804 31 657 57.0 130 454 87 811 33 669 56.3 135 456 86794 Example 22 10.5 739 52.3 142 512 77 639 of eighth 23 10.6 688 52.3142 468 86 622 invention 26  9.8 742 52.2 145 528 75 641 28 10.2 76251.3 147 530 74 652 30  9.9 686 55.2 132 462 85 803 32 10.3 662 57.4 132457 87 801 34  9.5 673 56.6 136 455 87 782 Compara- 35 730 37.4 140 51062 561 tive 36 681 41.0 131 454 71 799 examples 37 675 43.4 132 451 74804 Common QT: Quenching after heating to 900° C. and tempering at 550°C.; IQT: induction quenching and tempering at 170° C.; CQT:carburization quenching and tempering at 170° C.

As is clear from Tables 3 and 4, the samples according to the presentinvention are remarkably better in the critical upsetting ratio and thereduction of area, which are indicators of steel ductility, than thecomparative samples having the same carbon contents, and theirdeformation resistance and the hardness after the quenching andtempering are satisfactory.

Next, the steels listed in Table 5 were rolled into steel bars and wirerods 36 to 50 mm in diameter under the rolling conditions listed inTable 2, spheroidizing-annealed, and then hardened through quenching andtempering in the same manner as above. Table 6 shows the investigationresults of their structures and material properties. Comparing thesamples of Table 6 with the comparative samples of Table 4, the samplesaccording to the present invention are remarkably better in the criticalupsetting ratio and the reduction of area, which are indicators of steelductility, than the comparative samples having the same carbon contents,and their deformation resistance and the hardness after the quenchingand tempering are satisfactory.

TABLE 5 Rare earth Steel C Si Mn S Al N P O Cr Mo Nb Te Zr Mg Y element41 0.35 0.25 0.81 0.014 0.034 0.0054 0.015 0.0015 — — — — 0.0027 — — —42 0.44 0.24 0.80 0.008 0.028 0.0053 0.012 0.0009 — — — 0.0031 0.00180.0145 — — 43 0.45 0.20 0.84 0.011 0.031 0.0057 0.014 0.0012 — — — — —0.0164 — — 44 0.45 0.15 0.84 0.009 0.030 0.0048 0.015 0.0010 — — — — — —— 0.024 45 0.44 0.22 0.78 0.014 0.033 0.0060 0.015 0.0013 — — — 0.00250.0025 — — — 46 0.44 0.21 0.80 0.015 0.035 0.0053 0.014 0.0009 0.14 — —— 0.0020 — — — 47 0.35 0.25 0.82 0.016 0.030 0.0049 0.015 0.0009 1.100.16 — — — 0.0214 — — 48 0.34 0.24 1.80 0.015 0.032 0.0051 0.013 0.00101.08 0.16 — — 0.0034 — — — 49 0.34 0.25 0.78 0.009 0.035 0.0053 0.0150.0007 1.21 0.15 — — — — — 0.035 50 0.35 0.23 0.81 0.014 0.030 0.00530.013 0.0009 1.12 0.16 — 0.0030 0.0022 — — — 51 0.35 0.20 0.82 0.0160.033 0.0055 0.014 0.0010 1.05 0.17 — 0.0028 0.0024 0.0194 — — 52 0.190.24 0.79 0.013 0.032 0.0141 0.015 0.0010 1.11 0.17 — — 0.0020 — — — 530.20 0.21 0.81 0.011 0.030 0.0139 0.012 0.0014 1.21 — — — — 0.0178 — —54 0.19 0.25 0.80 0.014 0.030 0.0150 0.013 0.0012 1.21 — 0.021 — 0.0021— — — 55 0.21 0.20 0.85 0.011 0.034 0.0161 0.013 0.0011 1.13 0.16 0.021— — 0.0172 — — 56 0.20 0.22 0.81 0.008 0.035 0.0147 0.014 0.0014 1.100.17 0.025 — — — — 0.028 57 0.45 0.24 0.82 0.014 0.036 0.0048 0.0140.0009 0.12 — — — — — 0.016 —

TABLE 6 Structure and properties of bar Structure and prop- or wire roderties after spheroid- Hardness izing annealing difference Degree Degreebetween of sphe- of sphe- Area surface γ grain roidized roidized Roll-percentage Surface Center layer and size struc- struc- Refer- ing offerrite layer portion center number of ture of ture of Classifi- enceSteel condi- in surface hardness, hardness, portion, surface surfacecenter cation symbol No. tion layer, % HV HV HV layer layer portionRange ≦10% ≧20% ≧ No. 8 ≦ No. 2 ≦ No. 3 specified in the presentinvention Example 41 41 I 4 278 214 64 of fourth 42 45 I 0 284 204 80invention 43 46 I 0 282 201 81 44 47 I 0 321 227 94 45 52 I 0 339 239100  Example 46 44 I 0 291 202 89  9.7 of fifth 47 49 I 0 324 227 9710.9 invention 48 51 I 0 322 227 95 11.4 49 53 I 0 374 254 120  10.8 5056 I 0 337 238 99 11.8 Example 51 42 I 0 289 203 86 1 2 of seventh 52 50I 0 312 227 85 1 2 invention 53 55 I 0 340 241 99 1 2 Example 54 45 I 0291 202 89 1 2 of eighth 55 48 I 0 312 223 89 11.2 1 2 invention 56 54 I0 352 241 111  1 2 57 57 I 0 291 201 90  9.9 1 2 Structure andproperties after spheroidizing annealing Ferrite grain size Surfacenumber Defor- Surface Reduc- hardness Refer- of mation Critical layerTensile tion after QT, HV Classifi- ence surface resistance, upsettinghardness, strength, of area, Common cation symbol layer MPa ratio, % HVMPa % QT IQT CQT Range ≧ No. 8 specified in the present inventionExample 41 688 52.4 137 469 85 621 of fourth 42 740 5.26 143 514 78 642invention 43 736 52.5 140 513 78 274 44 758 50.8 145 528 72 285 45 67558.8 138 449 86 Example 46 736 52.0 143 521 76 639 of fifth 47 759 50.7142 532 73 652 invention 48 758 51.1 144 528 74 294 49 683 55.4 135 45985 800 50 679 57.7 138 455 87 811 Example 51 741 52.8 144 514 78 640 ofseventh 52 758 51.7 146 532 73 276 invention 53 675 58.0 137 454 89 792Example 54 10.0 741 52.7 145 514 76 643 of eighth 55 10.4 780 51.8 145532 75 287 invention 56  9.8 681 56.1 135 457 88 810 57 10.1 735 53.1145 523 77 642 Common QT: Quenching after heating to 900° C. andtempering at 550° C.; IQT: induction quenching and tempering at 170° C.;CQT: carburization quenching and tempering at 170° C.

Industrial Applicability

A steel bar or wire rod for cold forging according to the presentinvention is a steel bar or wire rod for cold forging excellent inductility after spheroidizing annealing, capable of preventing the steelmaterial from cracking during cold forging, which cracking hasconventionally constituted a problem in the cold forging afterspheroidizing annealing. As the present invention makes it possible tomanufacture forged machine components requiring heavy working by coldforging thanks to the above, it brings about remarkable advantages insignificantly enhancing productivity and saving energy.

1. A steel bar or wire rod for cold forging excellent in ductility afterspheroidizing annealing, characterized by: consisting of a steelcontaining, in mass, 0.1 to 0.6% of C, 0.01 to 0.5% of Si, 0.2 to 1.7%of Mn, 0.001 to 0.15% of S, 0.015 to 0.05% of Al and 0.003 to 0.025% ofN, and having the contents of P and O controlled to 0.035% or less and0.003% or less, respectively, with the balance consisting of Fe andunavoidable impurities; the area percentage of ferrite in themetallographic structure of the portion from the surface to the depth of0.15 of its radius being 10% or less, with the rest of the structureconsisting substantially of one or mote of martensite, bainite andpearlite; and the average hardness of the portion from the depth of 0.5of its radius to the center being lower than that of its surface layer(the portion from, the surface to the depth of 0.15 of the radius) by HV20 or more.
 2. A steel bar or wire rod for cold forging excellent inductility after spheroidizing annealing according to claim 1,characterized by further containing, in moss, one or more of: 3.5% orless of Ni, 2% or less of Cr and 1% or less of Mo.
 3. A steel bar orwire rod for cold forging excellent in ductility after spheroidizingannealing according to claim 1, characterized by further containing, inmass, one or more of: 0.005 to 0.1% of Nb and 0.03 to 0.3% of V.
 4. Asteel bar or wire rod for cold forging excellent in ductility afterspheroidizing annealing according to claim 1, characterized by furthercontaining, in mass, one or more of: 0.02% or less of Te, 0.02% or lessof Ca, 0.01% or less of Zr, 0.035% or less of Mg, 0.1% or less of Y and0.15% or less of rare earth elements.
 5. A teal bar or wire rod for coldforging excellent in ductility after spheroidizing annealing accordingto claim 1, characterized in that the austenite grain size numberaccording to Japanese Industrial Standard (JIS) in the portion from thesurface to the depth of 0.15 of its a radius is 8 or higher.
 6. A methodof producing a steel bar or wire rod for cold forging excellent inductility after spheroidizing annealing, characterized by:finish-rolling a steel material having a chemical composition specifiedin claim 1 while controlling its surface temperature to 700 to 1,000° C.at the exit from the final finish rolling stand, during hot rolling,and, after that, subjecting the roiled material to at least a processcycle of “rapidly cling the hot rolled malarial to a surface temperatureof 600° C. or below and subsequently making it recuperate by thesensible heat thereof so that the surface temperature becomes 200 to700° C.” or repeating the process cycle twice or more; and, by doing so,making the area percentage of ferrite in the structure of the portion ofthe steel bar or wire rod from the surface to the depth of 0.15 of itsradius 10% or less, and the rest of the structure consist substantiallyof one or more of martensite, bainite and pearlite, and also, formingthe structure in which the average hardness of the portion from thedepth of 0.5 of its radius to the center is lower than that of itssurface layer (the portion from the surface to the depth of 0.15 of theradius) by HV 20 or more.
 7. A steel bar or wire rod for cold forgingexcellent, in ductility characterized by: being a feel bar or wire rodaccording to claim 1 having undergone spheroidizing annealing; thedegree of spheroidized structure according to JIS G 3539 in the portionfrom the surface to the depth of 0.15 of its radius being No. 2 orbelow; and the degree of spheroidized structure in the portion from thedepth of 0.5 of its radius to the center being No. 3 or below.
 8. Asteel oar or wire rod for cold forging excellent in ductility accordingto claim 7, characterized in that the ferrite grain size number underJIS in the portion from the surface to the depth of 0.15 of its radiusis 8 or higher.